This invention relates to apparatus for measuring gas current in a vacuum tube and more particularly to apparatus for measuring gas current in a cathode ray tube.
In most vacuum tubes, the presence of gas is undesirable since the resulting ionization within the tube permits an excessive flow of grid current and has a deleterious effect on the cathode as well as other potentially damaging effects. In addition, the electrical characteristics of a gassy tube (having gas therein) are not constant and the operation of such is thus irregular. This is objectionable in cathode ray tubes where smooth response is essential for undistorted reproduction of images being shown on the screen.
The degree of vacuum can be accurately estimated by measuring the gas current (also referred to as the ionization current) in the tube. Since the gas current in ordinary vacuum tubes is relatively great, there is no need for a particularly sensitive meter in conventional tube testers to determine such current. Consequently, stray current due to high resistance leakage paths will have little effect on such a meter. However, in the case of cathode ray tubes where a very sensitive indicating device is required to measure the minute gas current, such stray currents will also be indicated and, unless they can be compensated for without affecting the sensitivity of the indicating device, the gas current reading will be unreliable. In most of today's highly evacuated cathode ray tubes, the leakage current will practically totally obscure the gas current.
One prior method of measuring minute gas currents in cathode ray tubes is described in U.S. Pat. No. 2,632,134 issued to Reid. In this method, the relatively large leakage current is compensated for by "zeroing" the indicating meter before the gas current is measured. This requires an operator to, in effect, measure and compensate for the leakage current of each individual cathode ray tube in order to obtain an accurate gas current measurement. For large quantities of cathode ray tubes, this procedure can be very time consuming.